Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations, the description carries the full behavioral burden. It usefully discloses two business rules — tokens are purchasable on any subscription including freemium, and they do not extend the subscription period — but says nothing about whether the price is quoted, tier-dependent, cached, or what permissions are needed.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loaded with the core purpose in the first sentence. The middle two sentences drift toward describing the top-up product rather than the tool itself, but they are short and add usable pricing context rather than padding.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With no output schema, the description does carry the return contract ('values' object), which is helpful. However, it omits the currency/unit of the returned cost and whether the price varies by subscription tier or purchase quantity — notable gaps for a pricing tool on a paywall product.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The tool takes zero parameters, so the baseline of 4 applies. The description correctly implies no input is needed, and the only variable (which subscription) is not passed as an argument.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource: returns the cost of a FUP token for a top-up. An agent can distinguish it from paywall__purchase_add_on_units (which executes the purchase) or paywall__customer_deduct_from_fup (which consumes tokens), but the description never names those siblings to confirm the distinction.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Usage is only implied — the text about purchasing additional tokens suggests this is a price-lookup that precedes purchase_add_on_units, but it never says 'call this before purchasing' or when not to use it. No prerequisites (auth, tier eligibility) are stated.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.